The present invention relates to a multicylinder jump-spark ignition engine, and more particularly to a novel operating method therefor and a control system therefor.
In general, in a jump-spark ignition engine, ignitability and stability of combustion are deteriorated in a low speed and low load condition since a proportion of remaining gas to newly supplied suction (fresh) air-fuel mixture increases. Accordingly, the amount of exhaust of unburnt hydrocarbon (HC) increases and combustion efficiency is deteriorated. The above trend is remarkably observed particularly in a 2-cycle jump-spark ignition engine because of disordered combustion and blow-out of newly supplied gas which are inherent to the 2-cycle spark-ignition engine.
Many approaches to resolve the above problem have been heretofore proposed, but known approaches are not sufficient in their effects, to reduce the amount of HC exhaust and cannot materially improve the combustion efficiency.
For example, in a known approach to the 2-cycle jump-spark engine, a laminar flow is fed to prevent the disordered combustion to improve the ignitability and the stability of combustion, or combustion is carried out in only selected ones of multiple cylinders in the non-load (low load) condition or an ignition timing is retarded to control shaft output to thereby increase a proportion of feed gas to reduce mis-ignition rate. Further, in order to prevent the blow-out of new gas, a timed fuel jet system has been known in which fuel is directly jetted into a scavenging port or a cylinder at an appropriate timing. Although those approaches have been effective in preventing either one of the disordered combustion and the blow-out of newly supplied gas, none of them could prevent both. Accordingly, they have been insufficient in the effects of reducing HC and improving the combustion efficiency. Although it is possible to prevent both the disordered combustion and the blow-out of the newly supplied gas by combining the timed fuel jet system with other approach, such a combined system has a practical problem because of its high cost.
It is therefore an object of the present invention to provide inexpensive and practical method and system which attain sufficient purification of exhaust gas of the multi-cylinder jump-spark ignition engine and improve the combustion efficiency thereof. To this end, in accordance with the present invention, based on the fact that the combustion in a so-called run-on phenomenon is the one which includes very small fluctuation and has a very small mis-ignition rate, the combustion in the non-load condition, which usually includes most serious problems in improving the purification of the exhaust gas and the combustion efficiency, is improved by the run-on phenomenon.
The run-on phenomenon per se has been known and it has also been known that the run-on phenomenon is apt to occur in the 2-cycle jump-spark ignition engine. However, the run-on phenomenon is normally apt to occur in a limited operation condition, that is, in a high speed, low load condition, and it has been said that the probability of the occurrence of the run-on phenomenon depends on an air-fuel ratio and a feed gas rate, and the run-on phenomenon usually does not occur in a low speed condition or when the feed gas rate is too low. This is associated with the fact that the run-on phenomenon can be considered as a kind of compression ignition combustion which occurs by natural ignition of newly supplied gas by the newly supplied gas being heated by heat of remaining gas and the temperature rise of the newly supplied gas during the compression thereof. Accordingly, in actual, it is not possible in a normal operation method to improve the combustion by causing the run-on phenomenon to occur in the non-load condition.
In the present invention, taking the causes of the occurrence of the run-on phenomenon into condition, a condition under which the run-on phenomenon can occur in the non-load condition is created to thereby cause the compression ignition combustion to occur in the non-load condition. More particularly, in the present invention, both suction flow and exhaust flow are throttled in the non-load condition to prevent the blow-out of the newly supplied gas, while gas flow in the cylinder is suppressed to enhance the mixing of the newly supplied gas and the remaining gas, and combustion is carried out in only selected ones of the multiple cylinders to increase the feed gas rate in the selected cylinders to thereby cause a kind of run-on phenomenon to occur in the non-load condition.
In implementing the above operation method, it is necessary, in practice, to smoothly shift the operation mode from the normal jump-spark ignition operation to the compression ignition operation, and an effective approach therefor is needed. It is also effective to develop the above operation method such that the combustion occurs in only selected ones of the multiple cylinders in the non-load condition to cause the compression ignition operation to occur in the selected cylinders, while the compression ignition operation is carried out in all of the cylinders in the low load condition. In this case, it is necessary to smoothly shift the operation mode in the other cylinders from the non-load condition to the low load condition. Similar measures are required when the supply of air-fuel mixture to all of the cylinders is blocked in deceleration operation and the operation mode is shifted to the compression ignition operation.
Accordingly, it is another object of the present invention to provide an effective operation method for a jump-spark ignition engine which assures smooth shift of operation mode. This is attained, based on the finding of the inventors that ignition time in the compression ignition operation is much earlier than that in the jump-spark ignition operation, by forcibly advancing the ignition time when the operation mode is shifted from the jump-spark ignition operation to the compression ignition operation to thereby smoothly carry out the shift of operation mode. Furthermore, when the operation mode is shifted from the air-fuel mixture feed blocking operation to the compression ignition operation, the jump-spark operation is temporarily carried out with the forcibly advanced ignition timing to assure smooth shift of the operation mode.
In the present specification, the term "suction flow" is intended to include scavenging gas flow in the 2-cycle engine and normal suction flow in other jump-spark ignition 4-cycle engines. The term "compression ignition operation" herein used means an operation in which air-fuel mixture is substantially spontaneously ignited in the course of compression and it includes an operation in which the jump-spark ignition is used.